1. Field of the Invention
The present invention relates to a brushless motor. More specifically, the present invention relates to a brushless motor providing a frequency signal generator and a pulse signal generator which can be utilized, for example, for a driving source of a rotary head of a video tape recorder.
2. Description of the Prior Art
One example of this kind of brushless motor is disclosed, for example, in the Japanese Patent Laid-Open No. 118055/1983 laid open on July 13, 1983. The brushless motor of this prior art comprises a disc-shaped rotor magnet, driving coils wound on a stator core facing the end surface of the rotor magnet through an axial air gap, and magnetic poles row for generating frequency signal magnetized in an axial direction of a rotary shaft is formed in an annular magnet provided under the rotor magnet. A magnetic poles row for generating pulse signals of a pitch different from the pitch of other portions, specifically a half of the pitch is formed at a portion of the magnetic poles row for generating a frequency signal. The two magnetic poles row of this annular magnet are both magnetized in an alternately opposite polarity in a circumferential direction. A frequency signal generating coil is formed so as to face the end surface of the annular magnet, and a portion of the frequency signal generating coil is formed as a coil for generating pulse signals so as to correspond to the pitch of magnetic poles for generating pulse signals.
In accordance with this prior art, a frequency signal responding to a rotation umber (r.p.m.) of the brushless motor is picked up from the frequency signal generating coil, and one or more pulse signals for each rotation of the brushless motor are picked up from the pulse signal generating coil.
However, in this prior art, the magnetic poles for generating pulse signals are formed at a portion of the magnetic poles in an alternately opposite polarity in the same direction, and therefore the frequency signal generating coil interlinks with magnetic flux from the magnetic poles for generating pulse signals likewise for the magnetic poles for generating frequency signal. Accordingly, the waveform of the frequency signal outputted from the frequency signal generating coil is disturbed. The study by the inventor of the present invention shows that in the prior art the waveform of frequency signal contains an amplitude modulation percentage of 2/(n-3).times.100[%] at a maximum. Here, symbol n represents the number of magnetic poles for generating frequency signal and the number of component wires of the frequency signal generating coil interlinking therewith. Thus, when a change takes place in the amplitude of the frequency signal, various inconveniences are produced in utilizing this frequency signal.